I am actually surprised to see your water PH can swing that dramatically before/after adding ammonia. Adding ammonia itself shouldn't have such impact on the water PH. In my case, PH went from 8.0 straight out of the tap to 7.8 in my aquarium.
Although there certainly is an "ideal" and "less ideal" as well as "stall" point of water PH for the bacteria, you shouldn't worry too much about it. As long as it is not at the "stall" point, your aquarium is cycling. The tap water in different areas have very different PH as well as hardness. Most people do not have the ideal PH for cycling, but they manage. The bottom line is, you can always do partial water changes whenever you feel like it is less than ideal.
Have you tested your water hardness? The PH swings too rapidly might mean extremely soft water. I didn't see such dramatic change until I get large amount of nitrate toward the end of the fishless cycling.
